Removing Ionospheric Corruption from Low Frequency Radio Arrays

The EOR is believed to be the time during which the intergalactic medium (IGM), which consisted entirely of neutral hydrogen, was reionized by the first stellar and quasistellar objects. As such, it represents an important era in the history of the universe; however, direct observations of the EOR are currently near impossible. Because of the Gunn-Peterson effect the EOR is difficult to study at wavelengths of less than about one micron (Carilli, 2005). As a result, it is best observed by radio through near-IR wavelengths. One particular candidate for observation, due to the high concentrations of neutral hydrogen prior to the EOR, is the 21-cm emission line of hydrogen. After accounting for the red shifting of the universe during the EOR (estimated to have occurred at some point in 17 6 ! ! z ) this corresponds to an observing frequency on the order of tens to hundreds of MHz, i.e. low frequency radio waves (Carilli, 2005). This poses a problem because the ionosphere causes large phase delays in low frequency radio waves. In particular, the phase delay of the signal caused by the ionosphere is characterized by the equation: